1. Field of the Invention
The present invention relates to a method for the production of single-oriented silicon steel sheet having low core loss.
2. Description of the Prior Art
Single-oriented silicon steel sheet (hereinafter referred to as oriented silicon steel) is used as nonpermanent magnetic material intended chiefly for the iron cores of transformers and other electric equipment and devices. It is required that the oriented silicon steel have a good magnetic flux density B.sub.10 value (the magnetic flux density in the rolling direction generated at a magnetic intensity of 1000 A/m) as the excitation property, and good core loss in W.sub.17/50 and W.sub.15/50 values (at an alternating current of 50 Hz, the core loss at a magnetic flux density of 1.7 T and 1.5 T).
Recently, with the rapid rise of energy costs, in order to conserve energy and resources, there has been strong demand for transformers and other electrical equipment with lower electrical power loss and higher efficiency.
Accordingly, there has been strong demand for oriented silicon steel core materials with better core loss.
The prior art relating to the improvement of the magnetic properties of oriented silicon steel discloses a method whereby a fundamental chemical composition of silicon steel contains mainly MnS or MnSe for the precipitation dispersion phase, and the silicon steel is subjected to two or more cold rolling steps including an intermediate annealing, as follows:
Japan Kokai Koho (Published Unexamined Patent Application referred to as Kokai hereinafter) 58(1983)-42727 discloses a fundamental composition containing 0.02.about.0.2% Cu, and attempting optimum of the precipitation dispersion phase by controlling the hot rolling temperature in order to improve the magnetic property.
Kokai 58(1983)-23407 discloses a fundamental composition containing 0.005.about.0.035% Sb and 0.04.about.0.18% Cu to attain a fine precipitation dispersion phase, and better magnetic properties are obtained by controlling the temperature of the intermediate annealing.
Kokai 52(1977)-94825 discloses that better magnetic properties are obtained by controlling the cooling rate of the intermediate annealing, and carrying out the aging in the final cold rolling process.
In the above prior art, magnetic properties are improved by altering to the chemical composition of the steel, by controlling the temperature of the intermediate annealing and the cooling rate, and by aging the steel in the cold rolling process, but the core loss value is still 1.08.about.1.39 w/kg (0.30 mm thick) at W.sub.17/50. Thus while core loss is reduced compared with previous methods, it is still not fully satisfactory, and there are still problems regarding the stable production thereof.
And Ser. No. 381,877 discloses a method for producing a single oriented electric magnetic steel sheet of a high magnetic flux density as follows: a silicon steel slab containing 2.5.about.4.0% Si, less than 0.085% C, 0.010.about.0.050% acid-soluble Al, 0.03.about.0.15% Mn, and 0.010.about.0.050% S is subjected to a hot rolling, to a precipitation annealing, to more than one final cold rolling in the range of a reduction 81.about.95% to produce a sheet with the final thickness, to a decarburizing, and finally to a finish annealing. In the above method, the precipitation annealing comprises heating the steel to a specified temperature in the range of a soaking temperature from 800.degree. C. to 1080.degree..about.1200.degree. C. at a rate of 2.degree.-10.degree. C./sec, holding it at the specified within 60 seconds, and thereafter cooling it. The cooling time is determined for 20.about.500 seconds till the steel reaches a specified temperature in the range of 900.degree..about.980.degree. C., then it is quickly cooled from the specified temperature to room temperature at a rate of more than 10.degree. C./sec.
A characteristic feature of the above invention consists in the following: a silicon steel containing 0.010.about.0.050% acid-soluble Al is subjected to an annealing immediately prior to the final cold rolling at a soaking temperature in the range of 1080.degree..about.1200.degree. C., and the final cold rolling is carried out with a reduction of 81.about.95%. Further, in the annealing prior to the final cold rolling, the steel is heated to a temperature above 800.degree. C. with a heating rate of 2.degree..about.10.degree. C./sec. During the annealing course, it is seen that Si.sub.3 N.sub.4 precipitated in the hot rolled steel sheet is decomposed while AlN is precipitated into an optimum size thereof.
In addition, the precipitated compound is prevented from growing too coarse by specifying the soaking time within 60 seconds, and a sufficient precipitation is realized by controlling the cooling from the soaking temperature to the 900.degree..about.980.degree. C., and subsequently it is quickly cooled to room temperature.
According to the method of U.S. Pat. No. 3,636,579, it is not always easy to obtain excellent magnetic properties by the variation of the AlN size after the precipitation annealing in accordance with the content of Al of the steel.
The above invention therefore proposed precipitation conditions for the formation of an optimum AlN hardly affected by the composition of a steel by an improvement of the annealing condition immediately prior to the final cold rolling.